A laser printer of a tandem type has been known, which separates the color of each pixel in image data into print colors (e.g., C, M, Y and K), thereby generating a plurality of sets of print color data, and which prints images of the respective print colors, one on another, thereby printing a multi-color image.
The laser printer of the tandem type has a plurality of drums, each for one print color. Arranged in parallel to one another, the drums can print images of respective colors at the same time. The printing speed is therefore high, which is an advantageous feature of the laser printer of this type. In some cases, however, the images of different colors may be displaced in position from one another. If the images are displaced in position from one another, the multi-color image printed will have blank regions and will inevitably be deteriorated in quality.
U.S. Pat. No. 7,362,467 describes a technique of first analyzing an image, that will appear being placed over a background image (which will be referred to as “upper image” hereinafter), and detecting a black region in the image analyzed, and expanding the background image partly into the black region to print the background image partly overlapped with the black region of the upper image, thereby suppressing generation of blank regions in the multi-color image.